Movies don’t always do a great job of depicting things. I imagine they might skew towards depicting practices that are common in southern California, or where the creative team grew up. If the cafeteria set on the back lot is shaped a certain way, re-dressing it to change to another style might not make sense for your budget.
It’s going to vary based on private/public, state, and then local school jurisdiction, wealth of the community, and wealth of the family.
My school experience had a mix of:
- Kids bringing their own lunch. This would often be a sandwich, and might include additional items like fruit (apple or banana) or single serving snack packets (chips, cookies). Sometimes there wouldn’t be a sandwich. Leftovers were rare, partially due to no microwaves being available for student use. Sometimes these items would be packed by a parent, or the kid might pack their own.
- Kids buying lunch. A hot lunch had a fixed fee per day and there were usually several items of questionable culinary/nutritional value to choose from. There were also cookies, chips, or milk available for purchase. There may have been more options here, I didn’t interact much with the paid lunches.
- Kids bringing money for lunch and choosing to pocket some or all of it, and just eating later.
- Kids not bringing money for lunch, and not bringing their own. This could be their choice, food insecurity, neglect, or any number of other reasons. Their friends may or may not feed them depending on the relationship.
- Trading items with friends. It doesn’t even have to be food for food.
- Various mixing and matching of the above depending on external factors. A kid might bring lunch then buy milk or a snack on top of their other items. Or they might beg their parents to buy them lunch once a week so that they could have pizza when it was available on Fridays.
Most tables would be a mix of these choices. Some tables might only have kids buying items, or only bringing items from home. In one friend group it might be considered lame to buy lunch, or lame to have something packed by a parent. In some groups and age brackets trading might be prolific, rare, or non-existent.
One egregious example of this is the battle near the end of the book. My memory is a bit fuzzy, so please forgive if I slightly botch the details.
Bilbo has no reason to care about the fight. The audience has no reason to care. So Tolkien doesn’t write the fight, and just knocks Bilbo unconscious at the start. One of the movies devotes a significant amount of runtime to the battle, despite the fact that audiences still have no reason to care. But you gotta have the big setpiece, right?